The discovery of the Pregnane X receptor (PXR) has led to a greater understanding of the fundamental mechanisms underlying upregulation of cytochrome P4503A and potentially other drug detoxification enzymes. We hypothesize that sequence variations (including single nucleotide polymorphisms, SNPs) in PXR or its cognate binding element (PXRE) in the CYP3A4 gene are central to human variation in both basal and inducible CYP3A expression. To critically study the importance of PXR SNPs to regulation of CYP3A a safe, reliable, and convenient assay will be needed to determine an individual s PXR genotype. Towards this goal we have cloned, mapped and are sequencing PXR. Our overall objectives are: (1) to identify sequence variations in PXR (or in the CYP3A4 PXRE) and determine their functional importance; (2) to correlate PXR SNPs to phenotypic variations in CYP3A; (3) to determine the overall frequency and pattern of sequence variations in PXR; and (4) to determine the functional importance of PXR SNPs and the molecular mechanisms leading to non- functional PXR in cellular and biochemical assays. Our pharmacogenomic approach takes advantage of our unique resources - genomic DNAs from CYP3A phenotyped human tissues and human study populations from numerous clinical trials with CYP3A substrates and inducers. Additionally, using cultures of human hepatocytes phenotyped for CYP3A induction we will prospectively identify the outliers in inductive drug response. The goal of our study is to use these defined patient populations simultaneously as a source of genotype-phenotype relationships and as a SNP discovery resource. Our hypothesis driven approach will immediately elucidate the functional consequences of PXR or PXRE SNPs to not only PXR function but CYP3A drug response as well and will serve as an important gentotype-phenotype/SNP pharmacogenomics model and provide further insights into mechanisms responsible for the highly variable pharmacokinetics of CYP3A metabolized drugs.